The system-wide transition known as the Merge is set to begin on September 6th, according to the Ethereum Foundation. During the Merge, Ethereum's existing execution layer (the Mainnet we use today) and its new proof-of-stake consensus layer, the Beacon Chain, are combined.
With the activation of the Bellatrix upgrade on September 6th, the starter gun for the merge will be sounded, setting in motion the complete process, which should be completed between September 10th and September 20th.
It aims to switch Ethereum's proof-of-work (PoW) mechanism to a proof-of-stake (PoS) mechanism in order to scale projects faster, more efficiently, and more securely.
According to the Foundation, the Merge process involves two steps. Firstly, the Bellatrix upgrade is set to go live on September 6th at 11:34:47AM UTC which is epoch 144896. According to data from Ethernodes, 73.5% of Ethereum nodes are now considered "Merge ready" ahead of the upcoming Bellatrix upgrade and prior to the official Merge, the Bellatrix upgrade is considered one of the last necessary steps.
The Paris upgrade will follow the Bellatrix upgrade, which will switch the Ethereum network to a PoS network. When the "Total Terminal Difficulty" (TTD) reaches 58,750,000,000,000,000,000,000, the Paris upgrade will be triggered. The TTD is the threshold difficulty of the last block mined in Ethereum before the merge; after TTD is met or exceeded, the next block will be produced by PoS.
In essence, TTD measures Ethereum network activity but is hard to describe succinctly. It is because of this TTD threshold that the Foundation cannot give an exact date for the switchover, as it depends on the ebb and flow of Ethereum usage after September 6th but it is estimated to occur around September 15th.